1. Field of the Invention
The invention relates to the field of vanadium extraction, and more particularly to a method for extracting vanadium from shale.
2. Description of the Related Art
Vanadium-containing shale is a very important vanadium resource for extraction of vanadium element in the form of V2O5. Vanadium in the vanadium-containing shale mainly exists in mica crystal structures in an isomorphous state, so that it cannot be directly soluble in water, alkali solution, or any diluted acids except for hydrofluoric acid. In view of such characteristics, high temperature baking and strong acid leaching are generally employed to destroy the mica crystal structure in the vanadium-containing shale for allowing the vanadium in the shale to be soluble in water, acid, or alkali solution and to be transformed into a vanadate for subsequent process.
Typical methods for extracting vanadium from the shale include:
1. Baking Using Sodium Chloride-Water Leaching-Ion Exchange-Vanadium Precipitation by Ammonium Salt
The method uses an additive including NaCl and Na2CO3, and a dosage of the additive exceeds 12 wt. %. A large amount of NaCl results in serious pollution of Cl2 and HCl. Besides, the leaching rate and recovery rate of vanadium is very low.
2. Blank Baking-Alkali Leaching-Extraction-Vanadium Precipitation by Ammonium Salt
The method uses a 2 mol/L NaOH solution as a leaching agent and requires high alkali consumption. A resulting leached solution contains a large amount of silicon impurities, which seriously influences the subsequent extraction process.
3. Direct Acid Leaching (or Pressurized Acid Leaching)-Extraction-Vanadium Precipitation by Ammonium Salt
The method employs a 4 mol/L sulfuric acid solution as a leaching agent and requires a high acid consumption. Furthermore, a large amount of alkali is necessitated to adjust the pH value so as to meet the requirement of the subsequent extraction process. Process of pressurized acid leaching-extraction for extracting vanadium consumes a large amount of energy, and uses 25-40 wt. % of sulfuric solution as the leaching agent. The process requires high acid consumption, and a resulting acid leached solution includes a high content of impurities, which seriously affects subsequent operations.